Ross : Classification of Diatoms 403 



row flanges running from the central nodule to the valve margin, projecting at 

 right angles to the valve surface but turned towards the apices at their free 

 edges. Their chambers are not close; they are approximately circular and are 

 closed on the inner side by a membrane with fine pores in triangular tesselation 

 and on the outer side by a membrane with a broad slit that runs across the 

 direction of the stria and is longer than the diameter of the main part of the 

 chamber. 



These observations not only confirmed that it is correct to separate the 

 species with a "bifid stauros" from Stauroneis and to associate the species with 

 amphoroid asymmetry and the symmetrical ones; they also suggested that 

 other species now grouped in Stauroneis might belong to separate genera. To 

 see whether examination of more species would provide evidence to confirm this, 

 S. acuta W. Sm., 5*. amphioxys Greg., and S. salina W. Sm. were examined under 

 the electron microscope, and more species will be as opportunity offers. The 

 stauros of the first two species appears under the light microscope to be broad 

 and narrow. The electron micrographs showed their structure to be as follows: 



(4) Stauroneis acuta (figures 7 and 8) has a broad and rather shallow stauros, 

 as in S. phoenicenteron and S. anceps, and the ultrastructure of its striae is 

 similar to that in those two species. 



(5) Stauroneis amphioxys (figures 9 and 10) has a broad and shallow stauros, 

 which extends for less than two-thirds of the width of the valve. Its striae 

 consist of distant circular chambers closed on the inner surface by a fine 

 membrane with pores in triangular tesselation. The chambers taper outwards, 

 i.e., they have the shape of truncated cones, but they have no membrane on 

 their outer side. 



(6) Stauroneis salina (figures 11 and 12) has a stauros that is rather deep 

 at the center of the valve and becomes narrower and shallower toward the 

 margin. The striae consist of distant circular chambers closed on the outer 

 side by an oblique parallel- sided slit that is slightly longer than the diameter of 

 the main part of the chamber and on the inner side by a fine membrane with 

 pores in triangular tesselation. The valve surface is depressed between one- 

 third and two-thirds of the distance from the raphe to the margin and through- 

 out this area the chambers in the striae are more distant than elsewhere. 



Taxonomic Implications 



Attention is here drawn to some similarities and differences in ultrastructure 

 that may have a taxonomic significance; not only the original observations 

 recorded above but also published micrographs of various species of Navic- 

 ulaceae are considered. The present state of our knowledge provides only a 

 very tenuous basis for taxonomic speculations; the justification for indulging 

 in these and putting them on record is that others may be stimulated to collect 

 further data that will tend to confirm or refute them. 



Stauroneis. Until recently the presence or absence of pseudosepta has been 

 treated as a character distinguishing sections within this genus (Cleve-Euler, 

 195vS). The close similarity which S. acuta, in which these are present, bears 

 in all other respects to S. phoenicenteron, in which they are absent, confirms the 

 view put forward by Hustedt (1959) that they are of httle taxonomic signifi- 

 cance. Also, Hustedt's (1959) contention that S. amphioxys (which he in- 



